1) Make sure you got target's speed right. More - and your torpedoes are ahead of target (or hit the bow). Less - and torpedoes either hit close to stern or simply pass it. Even at short range, 1 knot error may turn to be a miss if the target is short.
2) You don't "mark" with a scope. You should hold your scope completely still at the same bearing you calculated AOB for (that's your shooting bearing). Then you fire torpedoes when the desired part of the target crosses scope's center: thus, at this bearing this part of the ship is at calculated AOB, at calculated range, moving with calculated speed. It's just like each part was a different ship at the same course and speed.
3) The greater the gyro angle, the greater the error (I mean physical error in torpedo's final course). Try positioning your shot such that gyro angle is within -10..+10 degrees for best results. IIRC, you can't see actual angle with stock SH5 UI, but at least you can estimate it by looking at the solution line in TAI.
Torpedo speed is a tactical thing that has little to do with hitting when you've done the calculations right - TDC calculates gyro angle taking torpedo speed into account. Faster torpedo just needs less time to cross paths with target. Try switching torpedo speed while looking at solution line in TAI, you'll see how it jumps "closer" and "farther" away from shooting bearing (though at short range difference should be small), but the torpedo should still hit around the same spot. This paragraph assumes nor you nor your torpedoes are spotted. For example, if torpedo is spotted, target will try to evade it (change speed and/or course, which then implies that your calculations are no longer correct).
Take a look at
these Stoianm's "Hit and Run" videos. He uses more realism settings and mods, but shot plotting is exactly the same.